This invention relates to telephone systems and more particularly to an exclusion circuit contained in a subset of a key telephone system.
Exclusion circuits are incorporated in key telephone systems to prevent a subset from connecting a line that is already occupied.
Exclusion circuits may be automatic or manual. Some existing automatic exclusion circuits are dependent on central office or interface line voltage as a means of sensing line occupation. Others employ elaborate multiple relay switching arrangements that sense "A" lead conditions of a key telephone system. Manual exclusion circuits require remote manipulation to operate.
There are several shortcomings of the above-mentioned prior art exclusion circuits. Central office line voltage sensing circuits are dependent on central office line voltage. High or low central office voltage will cause the exclusion circuit to malfunction. Multiple relay switching arrangements are dependent on operation time of the relays. A multitude of relays may jeopardize the efficiency and reliability of the exclusion circuit, as well as increasing its cost and size.